Process for reducing anode corrosion in an acrylonitrile hydrodimerization cell



United States Patent PROCESS FOR REDUCING ANODE CORROSION IN AN ACRYLONITRILE HYDRODIMERIZA- TION CELL David W. Brubaker, Pensacola, Donald E. Danly, Cantonment, and Jim N. Maloney, Jr., Pensacola, Fla., assignors to Monsanto Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed July 26, 1965, Ser. No. 474,934

6 Claims. (Cl. 204--74) The invention relates to a method for reducing anode corrosion in an electrolytic cell employed to electrohydrodimerize acrylonitrile to adiponitrile. In a recent technological development a process has been developed whereby acrylonitrile can be electrohydrodimerizedto adiponitrile in a dually-compartmented cell. The compartments are a cathode and an anode compartment and are separated by a cation permselective membrane. A catholyte made up of, in part, water, acrylonitrile, a quaternary ammonium salt electrolyte, and reaction products, including adiponitrile, is continuously circulated through the cathode compartment. An anolyte composed of an aqueous solution of sulfuric acid is continuously circulated through the anode compartment. Sufficient electrical potential is established between the anode and cathode to produce unidirectional electrical current. Under the influence of such current acrylonitrile is electro'hydrodimerized at the cathode to produce adiponitrile which is then recovered.

The anode of the cell generally employed to accomplish acrylonitrile electrohydrodimerization is made from lead, lead'rsilver alloy, lead-antimony alloy, platinum, stainless steel, and other like materials. The most commonly employed anode material is a lead-silver alloy. In operation certain deleterious ions invade the anode compartment. Among these are included nitrate, perchlorate, and chlorate ions. As is also the case with perchloric and chloric acids, nitric acid is an oxidizing agent. These acids were found to be extremely corrosive of the anodes used in the electrolytic cell. The most concentrated deleterious ion is nitrate which forms nitric acid and is itself formed by the oxidation of acrylonitrile which has invaded the anode compartment and been oxidized at the anode.

At a current density of 0.15 ampere per square centimeter of effective anode it was determined that, employing anolyte which had been used in an electrohydrodimerization cell, a lead-silver anode Was corroding at the rate of 2.5 inches per year. When sulfuric acid which did not contain nitrate, perchlorate or chlorate ions was employed the corrosion rate dropped to 0.1-0.2 inch per year. The following table shows tabulated data indicating the effect which various deleterious ion levels in 0.6 N sulfuric acid have on the anode corrosion rate.

TABLE 1 Deleterious ions, Corrosion rate,

meq./liter: in./yr. 1.6 0.11

ice

life in "an electroyltic cell for electrohydrodimerizing acrylonitrile to adiponitrile.

Other objects will in part be obvious and will in part become obvious from the following descriptive material.

In general the above general object is accomplished by intimately contacting the sulfuric acid anolyte with a water insoluble organic solution of a high molecular weight amine. Thereafter, the water insoluble organic solution of a high molecular weight amine is separated from the anolyte.

1n the instant process when the anolyte containing impurities including nitric, perchloric, and chloric acids is contacted with a high molecular weight amine carried in an organic solvent such high molecular weight amine, either by neutralization or by ion exchange, selectively removes the deleterious ions from the sulfuric acid anolyte while at the same time removing only a very small quantity of the sulfuric acid itself. It would be possible to use the high molecular weight amines in the absence of a solvent carrier; however, such a procedure is not as effective as when a solvent carrier is employed.

As general criteria the amines here employed must be highly selective for nitric, perchlorate, and chlorate ions, must be substantially water insoluble, be low in cost, be highly miscible with low cost solvents, be capable of regeneration with common reagents, and be free from emulsion forming tendencies. These criteria are met by primary, secondary, and tertiary amines having a molecular weight from about 250 to about 500. In general, these amines are unifunctional, i.e., they contain only one ionizable group per molecule. Such amines can be used singly or as a mixture. Mixtures of amines are much more easily obtainable on a commercial basis. Two commercially available arnines are secondary amines having a molecular weight from about 350400 sold by Rohm and Haas Company under the trademark Amberlite LA1 and Amberlite LA2. Amberlite LA-l is a mixture of N dodecenyl-N-trialkylmethylamines having a molecular weight from about 351-393, a neutral equivalency of 380410, a freezing point below C., a pour point below 20 C. and a steady state solubility in IN sulfuric acid in parts per million of 15 and an acid binding capacity of 2.5-2.7 milliequivalents per gram. Amberlite LA2 is a mixture of N-lauryl-N-trialkylamine having a molecular weight from about 35 3-395, a neutral equivalency of 350-380, a freezing point below 10 C., an acid binding capacity of 2.6-2.8 milliequivalents per gram, and a steady state solubility in IN H 50 in parts per million of 0.

Examples of high molecular weight primary amines include 1-(3-ethylpentyl)-4-ethyloctylamine, l heptyloctylamine, and l-nndecyllaurylamine. Other secondary amines of high molecular weight besides those described before include bis-(1-isobutyl-3,5-dimethylhexyl) amine, di-n-decylamine, dilaurylamine, N-(l-undecyllauryl) laurylamine, N-benzyl-(l-nonyldecyl) amine, and N benzyl-l-undecyllaurylamine. A few of the many tertiary amines are trilaurylamine, tri-n-octylamine, didodenyl-n-butylamine, butylidaurylamine, and tribenzylamine.

The amnies here employed can be in the free base form, or in the salt form. It is possible to use the salt form and exchange an anion for the deleterious anions. However, by far the most practical procedure is to use the amines in the free base form to thereby neutralize and remove the deleterious acids found in the anolyte.

To achieve theoretically complete deleterious ion removal, it is obvious that at least an amount of high molecular amine stoichiometrically equivalent to the amount of deteterious ions present in the anolyte must be provided. In practice it has been learned that a small stoichio metric excess of amine should be used. Based on the nitrate ion alone, a ratio of 1.1 milliequivalents of amine to 1 milliequivalent of deleterious nitrate ion removes approxmiately 69 percent thereof. Upon increasing the milliequivalency ratio of amine to nitrate ion from 1.1:1 to 5.421 nitrate removal efficiency increases to about 86 percent. A further increase has only a very small effect. There fore, a milliequivalency ratio of not more than about 5.5 milliequivalents of amine to 1 milliequivalent of deleterious ions should be employed.

The organic solvents employed herein to produce organic solutions of amines must meet two general requirements. First, the solvent must be substantially water insoluble and second the amine employed must be highly soluble in the solvent. Among the generally useful organic solvents are included petroleum distillates, aromatic and aliphatic hydrocarbons, and high molecular weight alcohols. Specifically useful materials include benzene, xylene, and kerosene. It must be clearly understood, however, that there is a broad range of water insoluble materials whose usefulness should be apparent to one skilled in the art to which the present invention pertains.

The organic solvent plus amine soluble therein, which in contact with water forms an organic phase, must be only slightly soluble if not completely insoluble in acidic aqueous solutions, especially aqueous sulfuric acid solution. An aqueous to organic phase ratio greater than about :1 is undesirable and should be avoided.

The instant process can be practiced on a batch basis or it can be practiced in a continuous fashion. The main requirement is that whatever method be employed the aqueous and organic phases be thoroughly and intimately contacted. Various methods of contacting include counter flow in packed columns, contact in a vessel mechanically agitated, or in centrifugal contact apparatus. Many contact methods should be apparent to those skilled in the art.

Once the high molecular weight amines here employed have become loaded with deleterious ions they can then be recharged by contact with a number of alkaline materials including anhydrous ammonia, aqueous ammonia, sodium hydroxide solution, and other materials. If, in the poorer procedure, an anion exchange procedure is used then the proper recharging anion must be provided. Once the material 'has been recharged it is generally preferred that the organic solution containing amine be water-washed prior to its reuse for deleterious ion removal.

The following example is set forth to illustrate the invention. It is not intended that the invention be limited by its inclusion in any manner whatsoever.

Example Two volumes of a N organic solution of high molecular weight amine having the trademark Amberlite LA-2 (described hereinbefore) dissolved in xylene were contacted in a separatory funnel with 5 volumes of anolyte containing 500 milliequivalents per liter (2.5%) of sulfuric acid and 9.3 milliequivalents per liter of nitrate ion along with a small quantity of non-deleterious ions. After separating and analyzing the aqueous phase it was found that the nitrate level in the anolyte had been reduced to 2.9 millicquivalents per liter by this single stage extraction procedure. A loss of only 8% sulfuric acid was sustained. Test anode-panels of lead containing 1% silver were conatcted with the treated anolyte in a beaker test at a current density of 0.15 ampere per square centimeter of effective anode surface. The corrosion rate was found to be 0.2 inch per year as compared with a corrosion rate of 2.5 inches per year when untreated anolyte containing deleterious ions was employed. The contaminated organic solution of amine was regenerated with 0.27 volume of 0.46 N aqueous ammonia. Stoichiometrically this was of the amount required. Thereafter the regenerated amine solution was water washed three times using 0.1 volume of water per wash, thereby reducing the ammonium ion content to about 0.5 milliequivalent per liter. Organic amine solution recovery for reuse was about 99%.

The advantages of this invention can be summed up in the simple statementthe anode corrosion rate is reduced about 20 fold. This results in less down time for the electrolytic cell. Anode life is greatly extended with its attendant economic advantages. The process itself is simple and economical to operate, for the amines employed and the solvents therefor as well as the necessary equipment are all relatively inexpensive.

Although the invention has been described with reference to particular ranges, particular materials, and the like, it must 'be clearly understood that the invention in its many facets has very broad scope. Therefore, the invention should be broadly construed and should be only limited by the appended claims.

What is claimed is:

1. In a process for electrohydrodimerizing acrylonitrile to adiponitrile in an electrolytic cell having an anode equipped chamber through which an anolyte of aqueous sulfuric acid solution is continuously circulated and a cathode equipped chamber through which a catholyte including an aqueous quaternary ammonium salt solution, acrylonitrile, and reaction products is continuously circulated while an electrical potential is established between said anode and cathode sufficient to produce unidirectional current flow, a method for reducing corrosion of said anode by removing deleterious acids from said anolyte comprising, in combination, the steps of:

(a) intimately contacting said anolyte with a water insoluble organic solution of a high molecular weight amine; and

(b) separating said organic solution of high molecular weight amine from said anolyte.

2. The process of claim 1 wherein the amine has a molecular weight from 250 to 500.

3. The process of claim 1 wherein the amine is a secondary amine having a molecular weight from 350-400.

4. The process of claim 1 wherein the quantity of amine employed is sufficient to produce a ratio of not more than 5.5 to 1 amine equivalents per deleterious acid equivalent.

5. The process of claim 3 wherein the secondary amine is at least one N-dodecenyl-N-trialkylamine having a molecular weight from 351 to 393.

6. The process of claim 3 wherein the secondary amine is at least one N-lauryl-N-trialkylmethylamine having a molecular Weight from 353 to 395.

References Cited UNITED STATES PATENTS 2,065,112 12/1936 Bottoms 23-2 2,460,259 l/ 1949 Kahler 117-97 3,193,480 7/1965 Baizer et al. 204-73 3,335,162 8/1967 Campbell et a1 260459 HOWARD S. WILLIAMS, Primary Examiner. H. M. FLOURNOY, Assistant Examiner. 

1. IN A PROCESS FOR ELECTROHYDRODIMERIZING ACRYLONITRILE TO ADIPONITRILE IN AN ELECTROLYTIC CELL HAVING AN ANODE EQUIPPED CHAMBER THROUGH WHICH AN ANOLYTE OF AQUEOUS SULFURIC ACID SOLUTION IS CONTINUOUSLY CIRCULATED AND A CATHODE EQUIPPED CHAMBER THROUGH WHICH A CATHOLYTE INCLUDING AN AQUEOUS QUATERNARY AMMONIUM SALT SOLUTION, ACRYLONITRILE, AND REACTION PRODUCTS IS CONTINUOUSLY CIRCULATED WHILE AN ELECTRICAL POTENTIAL IS ESTABLISHED BETWEEN SAID ANODE AND CATHODE SUFFICIENT TO PRODUCE UNIDIRECTIONAL CURRENT FLOW, A METHOD FOR REDUCING CORROSION OF SAID ANODE BY REMOVING DELETERIOUS ACIDS FROM SAID ANOLYTE COMPRISING, IN COMBINATION, THE STEPS OF: (A) INTIMATELY CONTACTING SAID ANOLYTE WITH WATER INSOLUBLE ORGANIC SOLUTION OF A HIGH MOLECULAR WEIGHT AMINE; AND (B) SEPARATING SAID ORGANIC SOLSUTION OF HIGH MOLECULAR WEIGHT AMINE FROM SAID ANOLYTE. 